This invention relates to thermoplastic resin compositions and more particularly is concerned with polycarbonate resin mixtures having extraordinary resistance to environmental stress crazing and cracking.
Aromatic carbonate polymers are well known, commercially available materials having a variety of applications in the plastics art. Such carbonate polymers may be prepared by reacting a dihydric phenol, such as 2,2-bis(4-hydroxyphenyl)-propane, with a carbonate precursor, such as phosgene, in the presence of an acid binding agent. Generally speaking, aromatic polycarbonate resins offer a high resistance to the attack of mineral acids, may be readily molded, and are physiologically harmless as well as stain resistant. In addition, such polymers have a high tensile and impact strength (except in thick molded sections), and a dimensional stability surpassing that of other thermoplastic materials. However, in certain applications, the use of aromatic polycarbonate resins is limited because they exhibit severe environmental stress crazing and cracking. "Environmental stress crazing and cracking" refers to the type of failure which is hastened by the presence of organic solvents such as, for example, gasoline, acetone, heptane and carbon tetrachloride when such solvents are in contact with stressed parts fabricated from aromatic polycarbonate resins. The most significant effect is a loss in impact strength and also an increase in brittle type failure. Contact with such solvents may occur, for example, when parts are used under the hood of automobiles, or near the gasoline filler ports thereof, or when solvents are used to clean or degrease stressed parts made from polycarbonate resins.
At present, no entirely satisfactory means is available for reducing environmental stress crazing and cracking of polycarbonate resins, although a variety of methods have been proposed.
In Goldblum, U.S. Pat. No. 3,431,224, assigned to the same assignee as this application, for example, it is proposed to add modifiers to polycarbonate, in certain proportions, the modifiers comprising at least one member of the class consisting of polyethylene, polypropylene, polyisobutylene, a copolymer of ethylene and an ethyl acrylate, a copolymer of ethylene and propylene, a cellulose ester, a polyamide, a polyvinyl acetal, an alkyl cellulose ether, and a polyurethane elastomer. While the results with such modifiers are generally excellent, in thin sections, e.g., 1/8 inch, it has been found, as will be shown later herein, that the weld line strength is significantly improved by the compositions of this invention.
Another modifier proposed to be added to polycarbonate is reported in Research Disclosure No. 20810, Dow Chemical Company, August 1981. Data are provided showing that polycarbonates modified with a linear low density polyolefin, namely, ethylene/octene-1 copolymer, provide good impact strength at increased part thickness. There is no suggestion therein that such a modifier will enhance resistance to environmental stress crazing and cracking, and, as will be shown hereinafter, soaking a composition modified with a linear low density copolymer of ethylene and butene-1, even in thin sections, causes the impact strength to deteriorate substantially and results in brittle failure. Still other modifiers have been proposed for impact strength improvement, but none of them provides optimum environmental stress crazing and cracking resistance--applicant's earlier filed commonly assigned U.S. patent applications, Ser. No. 313,431, filed Oct. 21, 1981, being expressly mentioned in this connection. Ser. No. 313,431, describes polycarbonates modified with a linear low density polyolefin (LLDPE), e.g., a copolymer of ethylene and a small amount of butene-1. Such compositions process well and are toughened, but there is no disclosure of solvent resistance and, as will be shown later herein, the LLDPE's alone do not provide enhanced resistance to environmental stress crazing and cracking, even in thin sections.